Titrations of non-ionic surfactants with sodium tetraphenylborate using simple potentiometric sensors

Abstract

A method is described for the determination of non-ionic surfactants containing poly(oxyethylene) chains with sodium tetraphenylborate, based on the precipitation of ternary compounds in the presence of bivalent metal ions with an ionic radius greater than 100 pm [barium(II) salts preferred]. Titrations were monitored potentiometrically with a simple poly(vinyl chloride) membrane coated aluminium wire electrode plasticised with 2,4-dinitrophenyl octyl ether. The value of the potentiometric titration break depended on the length of the poly(oxyethylene) chains and on the type of cation present in excess. Stoicheiometric ratios (number of oxyethylene units per titrant molecule) could be corrected for compounds containing shorter chains on the basis of high-performance liquid chromatographic analyses. The results of 52 sets of titrations indicate that an average of 5.2 oxyethylene units correspond to one tetraphenylborate ion, i.e., 10.4 units to each Ba²⁺ ion. The possible conformation of the ternary complexes is discussed. For the analysis of detergents or other samples of practical importance, the establishment of empirical titration equivalents is recommended.